In the photographic industry the need to protect an imaging element from dirt and dust, scratches and abrasion, and deposition of stains has long been recognized. Many studies have been focused on the prevention of dirt and dust attraction through the use of antistatic layers in imaging elements. Recent improvements in protective overcoats have reduced the propensity for imaging elements to be scratched or abraded during manufacture and use. Far less attention has been given to improving the stain resistance of imaging elements.
The treatment of articles such as textiles and food containers with soil and stain resistant compositions is well known. For example, U.S. Pat. Nos. 3,574,791 and 3,728,151 disclose block or graft copolymers which have two different segments, one of which is highly fluorinated and oleophobic and the other of which is hydrophilic. U.S. Pat. No. 4,579,924 describes fluorochemical copolymers useful as paper making additives which impart oil and water repellancy and food stain resistance to ovenable paperboard food containers. U.S. Pat. No. 5,350,795 describes aqueous and oil repellent compositions which cure at ambient temperature. The compositions comprise an aqueous solution or dispersion of a fluorochemical acrylate copolymer and a polyalkoxylated polyurethane having pendant perfluoroalkyl groups. U.S. Pat. No. Re. 34,348 discloses stain resistant compositions containing fluorinated polymers derived from acrylamide-functional monomers. Fluorocarbon containing coatings for hard tissue and surfaces of the oral environment that reduce staining and adhesion of bacteria and proteinaceous substances are described in U.S. Pat. Nos. 5,662,887 and 5,607,663.
U.S. Pat. No. 4,229,524 describes copolymers containing perfluoroalkyl groups which can be used in a layer of a photosensitive material to improve antistatic and adhesion properties of the layer.
For a photographic element the requirements for a stain resistant overcoat are rather unique. The stain resistant layer must not effect the transparency, color, or other imaging properties of the film. The application and curing of the stain resistant coating must be compatible with the photographic element manufacturing process. The overcoat layer must be able to provide stain resistance when applied as a submicron-thick layer. In addition, the stain resistant overcoat should have excellent physical properties including resistance to scratch, abrasion, ferrotyping and blocking.
A wide variety of substances may adsorb onto or absorb into either the front or back surface of photographic elements and cause a permanent stain that degrades image quality. The deposition of these stain causing substances onto a photographic element may occur in many different ways. For example, dirt, fingerprints, and grease may be deposited onto the photographic element during handling. A photographic element may be stained when it comes in contact with a dirty surface or as a result of an accidental spill from, for example, a liquid drink such as coffee or soda. Other stains may be deposited onto a wet photographic element during film processing. For example, a tar-like material which is derived mostly from polymeric oxidized developer and which may be present at the surface of or on the walls of film processing solution tanks may be deposited during film processing. This tar may adhere to or diffuse into the surface layer of the photographic element and cause an extremely difficult to remove, brown-colored stain.
Film processor tar deposits and stain have been a particular problem with some protective overcoats used on motion picture film. In U.S. Pat. No. 5,786,134 and copending commonly-assigned, now U.S. Pat. Nos. 5,962,207 and 5,952,165 are described improved motion picture films that resist tar pickup. In these patent applications, the backside of the films contain a stain resistant layer applied over the protective overcoat. The protective overcoat is, in turn, applied over an electrically conductive layer. The present invention relates to eliminating tar pickup during processing and various problems associated with the generation of static charge by providing a new protective overcoat. This protective overcoat is obtained by the coating and subsequent drying of a coating composition containing a polyurethane, an interpolymer containing at least two different segments; one of which is fluorinated and therefore oleophobic, the other of which is hydratable, and an electrically conductive agent. Thus, in the present invention, stains such as tar deposits and problems associated with the generation of static charge are eliminated without the need to coat the two additional layers of the prior art. Further, the protective overcoat of the invention does not degrade the transparency, frictional characteristics, or other physical properties of the photographic element, and may be applied from solvent or aqueous media at low cost.